Frequency dispersive antenna applied in particular to a meteorological radar

ABSTRACT

The invention relates to a frequency dispersive antenna comprising radiating waveguides divided into three legs. The indirect angle between the first and the second leg is greater than or equal to 90 degrees and less than 180 degrees, the direct angle between the second and third leg being greater than or equal to 90 degrees and less than 180 degrees. The antenna comprises a feed waveguide comprising a stack of elements divided into three adjacent legs, the direct angle between the first and the second leg being greater than or equal to 90 degrees and less than 180 degrees, the indirect angle between the second and the third leg being greater than or equal to 90 degrees and less than 180 degrees. In particular, the invention applies to an airborne antenna suitable for the detection and for the pinpointing of meteorological phenomena.

The invention relates to a frequency dispersive antenna. In particular,the invention applies to an airborne antenna suitable for the detectionand for the pinpointing of meteorological phenomena.

Airborne meteorological radars comprise for the most part an antennawhose scanning in bearing and in elevation is carried out mechanically.The inertia of the antenna as well as the desired degree of agility ofthe beam influence the choice of the motors included in the scanningmechanisms. Such mechanisms may on account of their complexity becomeparticularly expensive.

In order to produce a less expensive antenna, it may in particular beopportune to resort to electronic scanning, at least in elevation,instead of the conventional scanning mechanisms. Specifically, theanalysis of the short and long range meteorological domain requires onlya few degrees of scanning in elevation, typically plus or minus 3degrees. In the case where the scanning in bearing is ensured by aconventional mechanism over plus or minus 90 degrees for example, asingle bearing scan is required to analyse the whole of themeteorological domain. The performance constraints on the motorsperforming the scan are considerably reduced without however degradingthe scanning performance of the antenna.

The conventional solutions based on RF phase shifters implemented in anantenna with electronic scanning are hardly suitable for the design ofan inexpensive antenna. Specifically, to obtain a reception patternwhere the sidelobes and diffuse lobes of the signal received by theantenna are lower by at least 30 dB with respect to the radiationmaximum, the required number of phase shifters becomes prohibitive interms of cost. Furthermore, the electrical consumption of an antennacomprising RF phase shifters is high, thereby complicating integrationin an aircraft where the electrical and thermal conditioning resourcesare limited.

Moreover, the reliability of such an antenna with electronic scanningturns out to be particularly sensitive to the failure rates of itscontrollable RF phase shifters. The predictive calculation of thereliability of the antenna being tricky since the rejection of thesidelobes and diffuse lobes degrade as a function in particular of thenumber of failed phase shifters and of the position of the latter, it isdifficult to guarantee a level of service.

The invention is in particular aimed at alleviating the aforesaiddrawbacks. In particular, but not exclusively, the invention is aimed atallowing an inexpensive meteorological radar antenna. For this purpose,the invention is aimed at an antenna comprising:

-   -   radiating waveguides, comprising at least three adjacent legs,        the angle between the first leg and the central leg being in the        clockwise sense greater than or equal to 90 degrees and less        than 180 degrees, the angle between the central leg and the        third leg being in the anticlockwise sense greater than or equal        to 90 degrees and less than 180 degrees, at least one coupling        slot being disposed on the rear face of the central leg of each        radiating waveguide;    -   at least one feed waveguide comprising a stack of elements, the        said elements comprising at least three adjacent legs, the angle        between the first leg and the central leg being in the        anticlockwise sense greater than or equal to 90 degrees and less        than 180 degrees, the angle between the central leg and the        third leg being in the clockwise sense greater than or equal to        90 degrees and less than 180 degrees, at least one coupling slot        being disposed on the front face of the central leg of each        element of each feed waveguide.        The coupling slot of each radiating waveguide is merged with the        coupling slot of an element of the feed waveguide. The central        leg of each radiating waveguide is crossed with the central leg        of an element of the feed waveguide. The variation of the        direction of pointing of the beam of the antenna in at least one        plane is obtained by varying the frequency of the wave guided by        the feed waveguide. The length of the feed waveguide between the        coupling slots of two adjacent radiating waveguides is greater        than the distance separating the coupling slots of these two        adjacent radiating waveguides.

In an advantageous manner, the first leg and the last leg of eachradiating waveguide are substantially parallel.

Advantageously, the first leg and the last leg of each element of thefeed waveguide are substantially parallel.

According to an aspect of the invention, the elements of the feedwaveguide are positioned in a plane parallel to the radiatingwaveguides.

In an advantageous manner, the elements of the feed waveguide can beflat slot waveguides. Likewise, the radiating waveguides can be forexample flat slot waveguides.

According to yet another aspect of the invention, the feed waveguideoperates in progressive mode.

According to yet another aspect of the invention, the radiatingwaveguides operate in resonant mode.

The antenna can in particular be used in a radar suitable for thedetection and for the pinpointing of meteorological phenomena.

The invention has in particular the advantages that the structure of thesidelobes of the pattern of an antenna according to the invention is notaffected by the variation of pointing of the beam of the antennawhatever the frequency of the RF signal emitted in the operating band,that it allows a particularly compact embodiment depthwise and that itis simple to implement.

Other characteristics and advantages of the invention will appear withthe aid of the description which follows, offered in relation to theappended drawings which represent:

FIG. 1, a principle of electronic scanning implemented on a radarantenna;

FIG. 2 a, an antenna according to the invention seen face on;

FIG. 2 b, a radiating waveguide according to the invention;

FIG. 2 c, an antenna according to the invention seen in profile;

FIG. 2 d, an element of the feed waveguide according to the invention;

FIG. 2 e, a detail of the feed waveguide according to the invention;

FIG. 3, a detail of the coupling between a radiating waveguide and anelement of the feed waveguide.

The following description takes in particular as example an airbornemeteorological radar comprising an antenna carrying out elevationalelectronic scanning of its beam. However, the invention can applyequally to any antenna whose beam scan is performed in at least oneplane in a nonmechanical manner.

FIG. 1 illustrates a principle of electronic scanning implemented on aradar antenna. The electronic scanning of the beam formed by the radarantenna can be obtained by phase shifting with respect to one anotherthe emission and reception pathways of an array of slots on the surfaceof the antenna. FIG. 1 shows a radar antenna comprising slots 1 ensuringthe reception and the emission of RF signals. These slots 1 are made ona right feed waveguide 2. The right feed waveguide 2 makes it possibleto convey the electromagnetic waves generated and amplified upstreamtowards the assembly of slots 1. Reciprocally, the right feed waveguide2 makes it possible to conduct the electromagnetic waves received by wayof the slots 1 towards the upstream devices charged in particular withthe amplification, the processing and the interpretation of the receivedsignals. In FIG. 1, an axis 3 is disposed perpendicularly to the planein which the slots 1 are gouged. This axis 3 cuts the plane comprisingthe slots 1 at a point situated at the centre of the array of slots 1.The antenna forms a beam 5 in the elevational direction of pointingdesired with respect to the axis 3 and denoted θ. In the same plane isrepresented an inclined plane 4 perpendicular to the pointing directionθ. This plane 4, called the phase plane, represents the phase shift ofthe RF waves received or emitted as a function of the vertical positionof the slots 1 so as to carry out a pointing in the desired direction ofpointing θ. The above description illustrates the known principle ofelectronic scanning in elevation. The principle is identical forscanning in bearing.

Hence, to carry out electronic scanning, it is appropriate to modify thephase of the electromagnetic waves received or emitted by the slots 1 asa function of their relative position and of the wave plane 4corresponding to the desired direction of pointing θ. By varying thewavelength of the electromagnetic waves traversing the right feedwaveguide 2 in a given interval of frequencies, it is thus possible toproduce a wave plane 4 as illustrated in FIG. 1. The amplitude of thesevariations, and hence the angular sector covered by the antenna in agiven plane, is in particular dependent on the length of the right feedwaveguide 2 between the slots 1.

A wave traversing the right feed waveguide 2 has in particular ascharacteristic a wavelength λ₀. Two adjacent slots 1 are spaced apart bya distance δ, close to 0.7 times the wavelength λ₀ for example. In thecase of an antenna carrying out a scan in elevation, the slots 1 can bedisposed horizontally in line, each line forming a group spaced apart bythe distance δ, close to 0.7 times the wavelength λ₀ for example. Thelength L corresponds to the actual linear distance traversed by theelectromagnetic wave in the right feed waveguide 2 between two adjacentslots 1 or two adjacent groups of slots 1.

By the principle of electronic scanning, it is necessary to create aphase difference φ_(n) between the first slot 1 for example and then^(th), the antenna comprising on its surface n slots 1, numbered in thedescription from 1 to n. If the frequency of the electromagnetic wavetraversing the right feed waveguide 2 is denoted f and λ_(g) thecorresponding wavelength, the phase difference φ_(n) can be definedaccording to the following formula$\varphi_{n} = {\frac{2\pi}{\lambda\quad g} \cdot n \cdot {L.}}$

On the basis of this formula and of the amplitudes of the waves incidenton the n slots 1 by electromagnetic coupling, denoted {A_(n)}, it ispossible to obtain an estimation of the gain in emission and inreception G as a function of the direction of pointing θ given byapplying the formula${G(\theta)} = {\sum\limits_{n}^{\quad}{A_{n} \cdot {{\mathbb{e}}^{j{({{\frac{2\pi}{\lambda\quad} \cdot n \cdot \delta \cdot {\sin{(\theta)}}} - \varphi_{n}})}}.}}}$This equation admits a maximum, corresponding to the radiation maximum,or main lobe at θ₀ such that, if λ is the length of the wave in vacuo atthe frequency f considered and k an even or odd integer making itpossible to bring the sine back to between −1 and 1,${\sin\left( \theta_{0} \right)} = {\frac{\lambda}{\delta} \cdot {\left( {\frac{L}{\lambda\quad g} - \frac{k}{2}} \right).}}$can in particular be chosen by taking the integer value closest toL/(λ_(g)/2) i.e. for example $\begin{matrix}{k = {{E\left( {\frac{L}{\frac{\lambda\quad g}{2}} + \frac{1}{2}} \right)}.}} & \quad\end{matrix}$

The ratio $\frac{\lambda}{\lambda_{g}}$varying with frequency, it is possible to make the angle θ of pointingof the antenna vary by a few degrees by varying the frequency applied tothe feed waveguide 2 of the array of slots 1. The bigger the length Lbetween 2 slots 1 or groups of slots 1, the larger the available angularspan for a given frequency band.

The right feed waveguide 2 possesses in particular as characteristic acutoff frequency f_(c) of its fundamental mode. The frequency band Δf,in which the frequency of the wave guided in the right feed waveguide 2can change so as to modify the direction of pointing θ of the antenna,possesses a central frequency f₀. The wavelength in vacuo of the centralfrequency f₀ is denoted λ₀. If the electromagnetic wave traversing theright feed waveguide 2 has a frequency equal to the central frequencyf₀, we then obtain a direction of pointing of the beam equal to θ₀withrespect to the mechanical axis of the antenna. The angular excursion Δθin radians expressing the angular span available for a given frequencyband can be evaluated according to the formula:${\Delta\theta} = {{\frac{1}{{\delta \cdot \cos}\quad\theta_{0}}\left\lbrack {{L\frac{\left( \frac{\lambda_{0}}{\lambda_{c}} \right)^{2}}{\sqrt{1 - \left( \frac{\lambda_{0}}{\lambda_{c}} \right)^{2}}}} + {\frac{k}{2} \cdot \lambda_{0}}} \right\rbrack} \cdot {\frac{\Delta\quad f}{f_{0}}.}}$

FIG. 2 a shows an antenna according to the invention seen face on. Inthe case of an antenna carrying out a scan in elevation, the slots 1 canbe disposed horizontally in line, each line forming a group. The averagedistance between two adjacent groups, measured for example between therespective centres of gravity of the two groups, is substantially equalto the distance δ, close to 0.7 times the wavelength λ₀ for example. Agroup of slots 1 can for example be produced on a radiating waveguide10. The radiating waveguide 10 can for example be a flat slot guide. Byelectromagnetic coupling, a wave traversing the radiating waveguide 10is emitted by way of the slots 1. Reciprocally, by electromagneticcoupling, the incident electromagnetic waves received by way of theslots 1 are transmitted to the radiating waveguide 10.

In an embodiment, the radiating waveguide 10 operates in resonant mode.For this purpose, the radiating waveguide 10 terminates inshort-circuits at each end.

The antenna presented in FIG. 2 a comprises several adjacent radiatingwaveguides 10, arranged so as to form an antenna whose front face has asubstantially circular form. Also, the antenna could equally wellcomprise a different number of lines or else a totally different form,as for example a rectangular form. The radiating waveguides 10 can forexample be produced on the basis of flat slot waveguides, exhibiting arectangular cross section.

FIG. 2 b shows a radiating waveguide 10 according to the invention. Inan embodiment, the radiating waveguide 10 comprises at least threeadjacent legs: a first leg A, a central leg B and a third leg C. In thecentral part of the rear face of the leg B of the radiating waveguide 10is sliced a coupling slot 21. This coupling slot 21 is substantiallydisposed at the centre of the rear face of the leg B of the radiatingwaveguide 10 so that the contribution of each slot 1 to the formation ofa wave in the radiating waveguide 10 is equivalent. The lengthwisecoupling slot 21 can be substantially parallel to the legs B and C ofthe radiating waveguide 10. In other applications, it could be desirableto arrange this coupling slot 21 differently. Slots 1 are drilled on thefront face of the various legs A, B and C of the radiating waveguide 10.Several coupling slots 21 could for example be drilled if several feedwaveguides 2 were used.

In a particular embodiment, the legs A, B and C are substantially in thesame plane. The leg A forms with the leg B an angle π_(AB). The angleα_(AB) oriented in the clockwise sense is greater than or equal to 90degrees and less than 180 degrees. Likewise, the leg B forms with theleg C an angle α_(BC). The angle α_(AB) oriented in the anticlockwisesense is greater than or equal to 90 degrees and less than 180 degrees.In a particular embodiment, the legs A and C are substantially paralleland hence the angles α_(AB) and α_(BC) are substantially equal. Theexact values of the angles α_(AB) and α_(BC) are determined as afunction of the angle of the coupling slot 21 of the radiating waveguide10 with respect to the feed waveguide 2 and of the level ofelectromagnetic coupling desired. This embodiment can however begeneralized to the case where the legs A, B and C are not in one and thesame plane. In this case, the arrangement of the various legs A, B and Cas well as their respective orientation then remain identical to whatwas described above by projection in one and the same plane.

FIG. 2 c shows an antenna seen in profile according to the invention.The antenna in particular comprises a feed waveguide 11 positioned herelengthwise parallel to the radiating waveguides 10. Each radiatingwaveguide 10 is coupled electromagnetically at at least one point withthe feed waveguide 11. The feed waveguide 11 is positioned in the planeparallel to the plane in which the radiating waveguides 10 are gouged.Furthermore, this arrangement exhibits the advantage of rendering theantenna compact in terms of depth via the feed waveguide 11.

In an embodiment, the feed waveguide 11 operates in progressive mode.For this purpose, a suitable load 12 is positioned at an end of the feedwaveguide 11. Specifically, the wave feeding the guide is then notreflected by the end of the feed waveguide 11.

FIG. 2 d shows an exemplary embodiment of a feed waveguide 11 accordingto the invention. The feed waveguide 11 can in particular comprise avertical stack of elements 23, each element 23 of the feed waveguide 11being for example produced on the basis of flat waveguides, exhibiting arectangular cross section. In an embodiment, each element 23 of the feedwaveguide 11 comprises at least three adjacent legs: a first leg D, acentral leg E and a third leg F. On the rear face in the central part ofthe leg E of an element 23 of the feed waveguide 11 is sliced a couplingslot 21. The lengthwise coupling slot 21 can be substantially parallelto the legs D and F of an element 23 of the feed waveguide 11. Thecoupling slot 21 of an element 23 of the feed waveguide 11 is mergedwith the electromagnetic coupling slot 21 of a radiating waveguide 10.The thickness of an element 23 of the feed waveguide 11 can furthermorebe reduced so as to facilitate the mechanical implementation of the feedwaveguide 11 in the form of a serpentine coil.

In a particular embodiment, the legs D, E and F are substantially in thesame plane. The leg D forms with the leg E an angle α_(DE). The angleα_(DE) oriented in the anticlockwise sense is greater than or equal to90 degrees and less than 180 degrees. Likewise, the leg E forms with theleg F an angle α_(EF). The angle α_(EF) oriented in the clockwise senseis greater than or equal to 90 degrees and less than 180 degrees. In aparticular embodiment, the legs D and F are substantially parallel andhence the angles α_(DE) and α_(EF) are substantially equal. The exactvalues of the angles α_(DE) and α_(EF) are determined as a function ofthe angle of the coupling slot 21 of each element 23 of the feedwaveguide 11 with respect to the corresponding radiating waveguide 10and of the level of electromagnetic coupling desired. This embodimentcan however be generalized to the case where the legs D, E and F are notin one and the same plane. In this case, the arrangement of the variouslegs D, E and F as well as their respective orientation then remaincomparable with what was described above by projection in one and thesame plane.

FIG. 2 e shows a detail of the feed waveguide according to theinvention. The feed waveguide 11 comprises a stack of elements 23. Foreach element 23, on one of the ends, an opening 22 links the element 23with the previous element 23 in the stack. This opening 22 has a crosssection substantially identical to the cross section of the previouselement 23 in the stack. On the other end, another opening 22 links theelement 23 with the element 23 following in the stack. This otheropening 22 has one and the same cross section substantially identical tothe cross section of the element 23 following in the stack. The distancebetween two coupling slots 21 disposed on adjacent elements 23 is equalto a distance δ. The distance that the wave traverses in the feedwaveguide 11 has a length L between these two same coupling slots 21. Inan embodiment of the antenna according to the invention, this length Lis greater than δ. By way of example, for an antenna operating in band Xand comprising standard feed waveguides 11 in band X, whose verticaldistance δ between two coupling slots 21 is equal to 25 mm and whosebeam must be able to scan in elevation the zone lying between −3 degreesand 3 degrees with respect to the horizon, the length L that the wavetraverses in the feed waveguide 11 between these two coupling slots mustbe at least 157 mm.

FIG. 3 shows a detail of the coupling between a radiating waveguide 10and an element 23 of the feed waveguide 11. The coupling slot 21 of anelement 23 of the feed waveguide 11 is aligned with the electromagneticcoupling slot 21 of a radiating waveguide 10. The legs A and C of eachradiating waveguide 10 as well as the legs D and F of the element 23 ofthe corresponding feed waveguide 11 are substantially parallel.Moreover, the legs C and D are aligned. The legs A and F are aligned.The angle α_(DE) oriented in the clockwise sense is substantially equalto the opposite of the angle α_(AB) oriented in the clockwise sense.Likewise, the angle α_(EF) oriented in the clockwise sense issubstantially equal to the opposite of the angle α_(BC) oriented in theclockwise sense. The legs B and E are hence crossed. Specifically, thelongitudinal axis of the central part of the radiating waveguide 10 onwhich the coupling slot 21 is produced crosses the longitudinal axis ofthe central part of the element 23 of the feed waveguide 11.

In the case of a radar working in band X, the radiating waveguides 10can for example be flat slot waveguides of outside dimensions of about23 mm for the large side and between 5 and 10 mm inside dimension forthe small side, the length being determined by the number of slots 1.The thickness has no influence on the parameters dimensioning theantenna. The width of the feed waveguide 11 being 23 mm and the verticallength L of the feed waveguide 11 between two groups of slots 1 being157 mm, the scan obtained by varying over a span of 100 MHz thefrequency f of the wave guided in the feed waveguide 11 then liesbetween −3 degrees and 3 degrees. The structure of the sidelobes of theantenna pattern in elevation is not modified by the variation ofpointing of the beam as a function of frequency.

1. An antenna, comprising: radiating waveguides, having three adjacentlegs, an angle between the first leg and the central leg being in theclockwise sense greater than or equal to 90 degrees and less than 180degrees, an angle between the central leg and the third leg being in theanticlockwise sense greater than or equal to 90 degrees and less than180 degrees, a at least one coupling slot being disposed on the rearface of the central leg of each radiating waveguide; a feed waveguidehaving a stack of elements, said elements having three adjacent legs, anangle between the first leg and the central leg being in theanticlockwise sense greater than or equal to 90 degrees and less than180 degrees, an angle between the central leg and the third leg being inthe clockwise sense greater than or equal to 90 degrees and less than180 degrees, a coupling slot being disposed on the front face of thecentral leg of each element of each feed waveguide; the coupling slot ofeach radiating waveguide being merged with the coupling slot of anelement of the feed waveguide, the central leg of each radiatingwaveguide being crossed with the central leg of an element of the feedwaveguide, the variation of the direction of pointing of the beam of theantenna in at least one plane being obtained by varying the frequency ofthe wave guided by the feed waveguide, the length of the feed waveguidebetween the coupling slots of two adjacent radiating waveguides beinggreater than the distance separating the coupling slots of these twoadjacent radiating waveguides.
 2. The antenna according to claim 1,wherein the first leg and the last leg of each radiating waveguide aresubstantially parallel.
 3. The antenna according to claim 1, wherein thefirst leg and the last leg of each element of the feed waveguide aresubstantially parallel.
 4. The antenna according to claim 1, wherein theelements of the feed waveguide are positioned in a plane parallel to theradiating waveguides.
 5. The antenna according to claim 1, wherein theelements of the feed waveguide are flat slot waveguides.
 6. The antennaaccording to claim 1, wherein the radiating waveguides are flat slotwaveguides.
 7. The antenna according to claim 1, wherein the feedwaveguide operates in progressive mode.
 8. The antenna according toclaim 1 the radiating waveguides operate in resonant mode.
 9. Theantenna according to claim 1, wherein the antenna is used in a radarsuitable for the detection and for the pinpointing of meteorologicalphenomena.